問72
頻出度★☆☆
USBケーブル経由で周辺装置に電力を供給する方式はどれか。
平成26年度 秋期 ITパスポート試験 問72
テクノロジ系コンピュータ構成要素 › 入出力デバイス
This question's explanation and your answers so far.
解説
問 72
Incorrectア
Standby refers to a power-saving state (standby mode) in which a device waits with reduced power consumption while it is not being used; it is not a method that expresses where the power comes from. Standby (suspend) on a PC is a mechanism that keeps the memory contents while stopping the display and the hard disk so that the system can resume quickly. It is not about the power supply path, so it cannot be the answer to this question.
Incorrectイ
Self-powered is the method in which the peripheral device operates on its own power source (an AC adapter or a wall outlet, a built-in battery, and so on), that is, exactly the side that does not receive power from the USB cable. External hard disks, printers, and powered USB hubs fall into this category. The question asks about the method that supplies power through the cable, so this is the opposite choice and is wrong.
- ウ
正解 The correct answer is ウ, Bus power. In addition to signal lines, USB has power lines (VBUS and GND), so a host such as a PC can supply power to a peripheral device through the USB cable. This way of operating on power taken from the USB bus itself is called the bus power method. A mouse, a keyboard, a USB flash drive, or a small USB LED light has no AC adapter and works by simply connecting one cable, and that is because it is powered by bus power. This term corresponds exactly to the wording of the question, a method that supplies power to peripheral devices via a USB cable, so ウ is correct. Note also that the USB standard sets an upper limit on the current available from one port (about 500 mA for USB 2.0), so devices with high power consumption cannot run on bus power and need the self-powered method described next; keep this point in mind as well.
Incorrectエ
An uninterruptible power supply (UPS) is a device that supplies power from a built-in battery for a certain period of time when a blackout or a momentary voltage drop occurs, so that servers and similar equipment can be shut down safely. Its purpose is to back up the commercial power supply, which is a completely different concept from a method of supplying power to peripheral devices over a USB cable, so it is wrong.
ここがポイント
- There are two power supply methods for USB devices. Bus power means the host supplies power through the USB cable (the bus); self-powered means the device is supplied from its own external power source. Devices with low power consumption use bus power, and devices with high power consumption use the self-powered method.
- Tell the confusing terms apart: standby is a power-saving waiting state, and an uninterruptible power supply (UPS) is a device that supplies power from a battery during a blackout. Neither of them is a USB power supply method.
用語